If a client representative asks or instructs you to do something on site (of any value or lasting effect) get it in writing before doing it.
For example, if the scope calls for a certain size concrete slab, but the client on site asks for it to be made larger, get it in writing before proceeding.
The person you get the written instruction from must be authorized to instruct you (not just a site operator or employee, it should be the project manager or his representative, who should be documented).
